Trimethylolpropane (TMP) is prepared from n-butyraldehyde and formaldehyde. In one preferred process, base-catalyzed aldol reaction initially generates 2,2-dimethylolbutyraldehyde in a first reaction step which is then converted to a TMP-formate mixture by way of a Cannizzaro reaction. The TMP-containing mixture is typically extracted with an organic solvent, such as ethyl acetate providing an aqueous phase containing the formate and the organic phase comprising TMP. The solvent is separated and the crude TMP is purified by distillation. Typical processing is seen in U.S. Pat. No. 5,603,835 to Cheung et al., Comparative Example 1, Col. 7. See, also, U.S. Pat. No. 5,948,943 to Supplee et al.
Purification residue of the TMP production process includes various formals in which TMP equivalents are chemically bound. Such formals may include all or some of the following:
monocyclic TMP-formal (MCF); Formula I:

monolinear bis-TMP-formal (MBLF or TMP-BMLF); Formula II:[C2H5C(CH2OH)2CH2O]2CH2  (II)
Methyl-(monolinear)TMP-formal; Formula IIIC2H5C(CH2OH)2CH2OCH2OCH3  (II)
Methyl-(bislinear)TMP-formal; Formula IVC2H5C(CH2OH)2CH2OCH2OCH2OCH3  (IV)Also, typically present in the purification residue are substantial amounts of di-TMP and the cyclic formal of di-TMP; Formula V:
as well as relatively large amounts of TMP. TMP has a structural formula:
and di-TMP has a structural formula:

In many cases, the main components in the purification residue are TMP, di-TMP and linear bis-TMP-formal (MBLF) (Formula II). Cyclic formals are usually present in smaller amounts.
Because the residues contain substantial amounts of TMP, either physically mixed or bound in formal equivalents, various processes to recover TMP from residues have been developed.
U.S. Pat. No. 7,301,058 to Wartini et al. discloses a process for recovering TMP from purification residue using acid treatment in water. Increases in TMP levels of from −6.4% to 36.3% are reported in Table 1, Col. 9. DD 28 72 51 likewise refers to cleavage of TMP formals under acidic conditions followed by thermal treatment.
U.S. Pat. No. 6,265,623 to Moraweitz et al. relates to the reductive cleavage of linear and cyclic acetals, especially formals, in an aqueous medium containing a formate which takes place by hydrogenation with hydrogen in the presence of a heterogeneous hydrogenation catalyst at a pH value of less than 7 at a temperature of over 200° C. See, also, WO 97/01523. Under these reaction conditions, formaldehyde liberated from the formals is hydrogenated to methanol. A similar hydrogenation process of the TMP residue is disclosed in DE 10 2010 033 844. The aqueous solution from the hydrogenation step is freed from the catalyst and other solids and then treated with ion exchangers. From the collected solution, light ends are removed and then a TMP-enriched head fraction is recovered.
United States Patent Application Publication No. US 2002/0033325 of Ninomiya et al. teaches to add an acid to TMP purification residue and to add a scavenger for the formaldehyde liberated in the acid treatment. Said scavenger, such as hydroxylamine salts, avoids the formation of cyclic formals, such as the cyclic formal of TMP and the cyclic formal of di-TMP.
In U.S. Pat. No. 6,316,679 to Supplee et al. there is disclosed a process for treating TMP heavy ends residue with a strong acid in the presence of an alcohol formaldehyde scavenger. Increases in TMP content from 10% to 30% are reported. See Table III, Col. 6. Similar features appear in U.S. Pat. No. 6,096,905, also to Supplee et al.